Verification of the Structural Design Parameters for Unbound Layers of Finnish Road Structures

Approaches used for the structural design of road pavements are most often combinations of mechanistic and empirical elements, the latter ones of which take care of adaptation to the local ambient conditions and available construction materials. In Finland, the standard design approach is to use so called Oedemark’s bearing capacity concept, in which the overall stiffness of a road structure is designed to meet a target value set based on the number of equivalent standard axle loads (ESALs) during an expected service life period. In the meantime, the stiffness value for each structural layer material is estimated based on its granularity and location in road structure, including, thus, at least to some extent indirectly also the stress conditions under which each material is exposed to during a heavy vehicle loading. However, when the design stiffness values are compared to those determined based on back-calculated response measurement values from two extensively instrumented road sections using a 3D finite element model, it is evident that the standard design stiffness values for unbound layer materials are far too low for describing the true mechanical responses of the instrumented road sections. Therefore, they cannot be applied as such in any purely mechanistic pavement performance analyses and related service life estimates. More realistic values for structural design parameters are suggested in this paper.